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Before Present
Before Present (BP) years is a time scale used mainly in archaeology, geology and other scientific disciplines to specify when events occurred in the past. Because the "present" time changes, standard practice is to use 1 January 1950 as the commencement date (epoch) of the age scale, reflecting the origin of practical radiocarbon dating in the 1950s. The abbreviation "BP" has been interpreted retrospectively as "Before Physics"; that refers to the time before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, making dating after that time likely to be unreliable. In a convention that is not always observed, many sources restrict the use of BP dates to those produced with radiocarbon dating. Usage The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy. This usage differs with the recommendation by van der Plicht & Hogg, followed by the Quaternary Science Reviews, both of which requested that publications should use the unit "a" for year and reserve the term "BP" for radiocarbon estimations. Some archaeologists use the lowercase letters bp, bc and ad as terminology for uncalibrated dates for these eras. The Centre for Ice and Climate at the University of Copenhagen has proposed "b2k" as "years before AD 2000", based on the Greenland Ice Core Chronology 2005 (GICC05) time scale. Radiocarbon dating Radiocarbon dating was first used in 1940. Beginning in 1954, metrologists established 1950 as the origin year for the BP scale for use with radiocarbon dating, using a 1950-based reference sample of oxalic acid. According to scientist A. Currie Lloyd: "The Remarkable Metrological History of Radiocarbon Dating [II"] at Google Books (accessed 30 October 2019).}} The year 1950 was chosen because it was the standard astronomical epoch at that time. It also marked the publication of the first radiocarbon dates in December 1949, and 1950 also antedates large scale atmospheric testing of nuclear weapons, which altered the global ratio of carbon-14 to carbon-12. Radiocarbon calibration Dates determined using radiocarbon dating come in two kinds: uncalibrated (also called Libby or raw) and calibrated (also called Cambridge) dates. Uncalibrated radiocarbon dates should be clearly noted as such by "uncalibrated years BP", because they are not identical to calendar dates. This has to do with the fact that the level of atmospheric radiocarbon (carbon-14 or 14C) has not been strictly constant during the span of time that can be radiocarbon-dated. Uncalibrated radiocarbon ages can be converted to calendar dates by means of calibration curves based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as dendrochronology (dating on the basis of tree growth-rings) and stratigraphy (dating on the basis of sediment layers in mud or sedimentary rock). Such calibrated dates are expressed as cal BP, where "cal" indicates "calibrated years", or "calendar years", before 1950. Many scholarly/scientific journals require that published calibrated results be accompanied by the name (standard codes are used) of the laboratory concerned, and other information such as confidence levels, because of differences between the methods used by different laboratories and changes in calibrating methods. Other dating conventions *Ab urbe condita (AUC) *Anno Domini (AD) *After the Development of Agriculture (ADA) *Anno Hegiri (AH) *Anno Mundi (AM) *Before Christ (BC) *Before Common Era or Before Current Era (BCE) *Common Era or Current Era (CE) *Cosmic time *Holocene calendar (HE) *years ago (ya) See also *Anthropocene Footnotes Citations Category:Calendar eras Category:Geochronology